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Having just briefly looked at both in the OED there seems to me plenty of room for interchangeability in the way they are used.

Under 'retrospective' for example there is an instance of legislation having been passed which took effect from an earlier time. This would seem to me as though it should be 'retroactive'.

And interestingly both words carry meanings which include the other. Can anyone summarise the difference by way of simple examples as to their separate uses?

Edit

retrospective, adj. and n. View as: Outline |Full entryQuotations: Show all |Hide all Pronunciation: Brit. /ˌrɛtrə(ʊ)ˈspɛktɪv/ , U.S. /ˌrɛtrəˈspɛktɪv/ Etymology: < post-classical Latin retrospect-, past participial stem of retrospicere to look back (see retrospect v.) + -ive suffix. With the use as adjective compare French rétrospectif (1779), Spanish retrospectivo (1587). With the use as noun compare earlier retrospect n., retrospection n., and (with the specific use in sense B. 2) French rétrospective (1920 in this sense; 1919 in sense ‘screening of a series of old films’), Spanish retrospectiva (1872 in this sense). Compare earlier retrospectively adv., which may imply earlier currency of the adjective. N.E.D. (1908) also gives the pronunciation (rītrospe·ktiv) /riːtrəʊˈspɛktɪv/ . (Show Less) A. adj. Thesaurus » Categories »

  1. Chiefly Law. Taking effect from a date in the past; retroactive.

1660 F. Philipps Tenenda non Tollenda iii. 110 The new mode of making retrospective Acts of Parliament.

a1768 J. Erskine Inst. Law Scotl. (1773) I. ii. vii. §15 It hinders the confirmation from having that retrospective quality.

1788 Considerations submitted to House of Lords on Two East-India Bills 54 All Acts of the Legislature, are retrospective, as well as prospective.

1843 Macaulay Crit. & Hist. Ess. I. 465 Sentencing a man to death by retrospective law.

1873 Sat. Rev. 9 Aug. 36/1 It is a retrospective alteration of a contract by one of the parties to the detriment of the other.

1899 T. C. Allbutt et al. Syst. Med. VIII. 321 The amnesia, however, is not, so to speak, retrospective.

1934 Yale Law Jrnl. 44 360 The arguments on the legality of retrospective legislation have been impressive in variety, but unconvincing in substance.

1955 Times 28 June 6/5 The decision was retrospective to April 1, when the new rates came into operation.

2004 H. Kennedy Just Law (2005) iii. 82 The retrospective nature of the intended legislation makes it doubly wrong.

Above is meaning number one (or retrospective) from the OED. Clearly it encompasses the 'retroactive'. And some of the more recent examples are clearly ones where you would claim 'retroactive' were appropriate. It seems to me that in Briatain, at least, 'retrospective' has to some extent taken on the meaning of 'retroactive'.

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If you're wondering whether two words are interchangeable, and your dictionary offers one word as an alternate definition of the other, you can be quite sure that there is at least one context in which the words are interchangeable. – phoog Apr 4 at 22:38

While they sometimes may be used interchangeably, retrospective is primarily a looking quality while retroactive is primarily doing.

You can have a retrospective show of an artist's work (looking back over her career). You would not have a retroactive show.

Similarly you can have a retrospective analysis (reviewing past activities), but not a retroactive analysis.

When an action is intended to be effective based on an earlier period, the term retroactive is more often used (at least in the US). In effect, you are saying even though it happens now, we are treating it as if it happened then.

However, you may say we are taking action now and acting based on a retrospective computation. This is arguably different from a retroactive event. The former is treated as a present activity and the latter is treated as an activity that relates back to an earlier time (nunc pro tunc for the Latin/legal geeks).

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Note the editing of my OP. In Britain, at least, it would appear that retrospective takes on some of the meanings of retroactive. – WS2 Jan 30 '14 at 22:07

The true meanings are easy to divulge through their etymologies:

retrospective - c.1600, "a regard or reference" (to something), from Latin retrospectum, past participle of retrospicere "look back,"

Citation

retroactive - 1610s, from French rétroactif (16c.) "casting or relating back," from Latin retroact-, past participle stem of retroagere "drive or turn back," from retro- "back" (see retro-) + agere "to drive, set in motion" (see act (v.)). Related: Retroactively.

Citation

The key words here are "set in motion". Retroactive means there is an action. It means that something, such as a law, becomes effective at a date in the past. This is opposed to retrospective, which refers to analysing or looking back at something in the past.

A good example is retroactive memory loss, where the memory loss causes your memory to become blank for a few hours/minutes before the time you received a concussion. Retrospectively, you cannot remember what happened.

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Beware the etymological fallacy. – phoog Apr 4 at 22:44

You have asked for a summary of the distinction, and you have noted that some statements are constructed such that one of these terms might include the other, but they really do not do that. They just sometimes crop up in similar contexts.

One way to think about this is to consider that retrospective involves looking backwards at past events, as if they are fixed but in a complex context, with considerable consciousness of a gap of time.

If a certain decision is retroactive, it will systematically affect past events. Such ‘events’ will typically be abstract: it is hard to reverse a physical avalanche and start again, but we can reinterpret existing information in light of new discoveries, and move on from that theoretical point as if starting from way back.

We might retrospectively consider how the millions believing that the world is flat have affected the course of civilisation. That would be informative and interesting.

Separately and differently, this week we might retroactively decide to free from prison any militant flat-Earther incarcerated under the rather punitive We Hate Non-Scientists Act (1999). That makes no difference at all to those flat-Earthers imprisoned (alongisde homosexuals, scientists and others) under the You’re an Insult to God Act (1553), under which you will still get strapped to a cannon, disemboweled and thrown in the sea.

Retrospective means (pretty much literally) with hindsight, and is generally about creating new interpretation with the previously-unavailable breadth and/or detail of historical perspective.

Retroactive means influencing earlier actions, and is generally about modifying events (e.g. the passing of laws) that had been considered complete. Merriam-Webster suggests ‘effective from a particular date in the past’, but in fact a precise date might not be necessary.

Recent UK investigations have retrospectively reassessed the general impression that we all had of the celebrity Jimmy Savile. That has led to a (probably) useful addition to the public consciousness, but nothing can retroactively affect any of the events involved.

However, recent UK legislation (see this BBC discussion) has retroactively reopened many (many, many, many) instances of people getting mortgages and other financial products, and buying ‘payment protection’ insurance (PPI) policies (i.e. if you lose your job the insurance will support your payments) that were sold alongside them. It has been determined that many UK citizens have been unnecessarily ‘mis-sold’ PPI as if it was a requirement. Those purchases were entirely legal and above-board, in their own terms, until it was decided that this option had been sold as a requirement. Having cleared your mortgage under your own steam long ago, you might have concluded retrospectively that that PPI purchase had not paid-off, but that does not change anything empirical. The retroactive legislation, however, reaches into what we thought was history and acts upon it: it actively changes what we thought were established and fixed features of our environment. This example means that claims of mis-selling make companies liable for misconduct not only for new transactions, but also for the one (completely legal at the time) that you completed and forgot about long ago.

Retrospection does not do that. At least, not by itself. Retrospection looks at past events in light of information that I didn’t have then. You might usefully compare it to introspection, which again changes nothing except present ideas (and, similarly, might change again).

There is no real overlap between these two terms, except in them both relating to the rational appreciation of time passing.

I, too, am a UK speaker of English, and I would not say that there is not really any systematic overlap between the two meanings. Then again, you will know as well as I do that we are not short of journalists willing (for example) to assume that everything prefixed with ‘retro’ is basically just equivalently clever.

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Your answer doesn't seem to account for the many examples of "retrospective" for acts of parliament whose effect reaches back into thetimebefore they were passed. Have I overlooked something? – phoog Apr 4 at 22:47
    
@phoog Apologies for delay in responding... There might easily be divergent uses in specialised professional discourses. I can guess that in legal use the term 'retrospective' might be the qualifier that means 'encompassing past events, not just materialising now'. I agree with OP in saying, 'This would seem to me as though it should be "retroactive".' If legal discourse over 344 years uses the terms interchangeably, however, then perhaps in law looking professionally amounts to acting. OP asked about the possible distinct meanings, though, and one does one's best. – Captain Cranium Apr 9 at 13:02

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